Attach option for existing app

[huylg]

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hi @jonsamwell, at #250 we allow the app still run after the test is finished. so let us provide the option to attach the existing app. Avoid rebuilding and rerunning apps will save a lot of time. Let me know if you want to discuss more.

[jonsamwell]

If you are attaching to an already running app don't you need to provide the observatory url?

Could you remove your changes from the pubspec.lock file

Plus also target the integration_test__package_support branch as we are soon merging this to master

[huylg]

we can't restartAppBetweenScenarios while run test in runningAppProtocolEndpointUri. idk why, so attach option will be valuable. I will remove the change from pubspec.lock and target the integration_test__package_support but this pr should be merged in master too.